The present invention relates to a laser oscillating apparatus for generating a laser beam by introducing an electromagnetic wave from a waveguide into a laser tube through a plurality of fine slots formed in the waveguide wall and, for example, to a laser oscillating apparatus using a microwave as an electromagnetic wave for exciting a laser gas, an exposure apparatus using the same, and a device fabrication method using the same.
Recently, a so-called excimer laser has attracted attention as the only high-output laser which oscillates in the ultraviolet region. This excimer laser is expected to be applied to the electronic, chemical, and energy industries, particularly processing and chemical reactions of metals, resins, glass, ceramics, and semiconductors.
The principle of function of an excimer laser oscillator will be described below. First, laser gases such as Ar, Kr, Ne, F2, He, Xe, Cl2, and the like contained in a laser tube are excited by electron beam irradiation or discharge. Excited F atoms bond to inert Kr and Ar atoms in the ground state to generate KrF and ArF as molecules existing only in an excited state. These molecules are called excimers. Since excimers are unstable, they immediately emit ultraviolet rays and fall to the ground state. This phenomenon is called spontaneous emission. An excimer laser oscillator uses this light to amplify as an in-phase beam in an optical resonator constructed of a pair of reflecting mirrors and extract as a laser beam.
In the case of excimer laser emission, microwaves are used as a laser gas exciting source. Microwaves are electromagnetic waves having an oscillation frequency of a few hundred MHz to several tens of GHz. In this case, a microwave is introduced from a waveguide into a laser tube through a slot formed in the waveguide wall, thereby exciting a laser gas in the laser tube into a plasma.
It is, however, difficult to uniformize the radiation characteristic of an electromagnetic wave from a slot formed in a waveguide wall in an entire region over the slot. Usually, the distribution is a sinusoidal distribution in the slot long-axis direction or a similar distribution. That is, as shown in FIG. 13A, an electric field intensity distribution in the center along the long-axis direction of each slot is largest, and the field intensity distribution at the ends in the long-axis direction of each slot is smallest.
Additionally, an excited plasma has a property of concentrating to the center in the long-axis direction of each slot with respect to the microwave field intensity distribution. This promotes the nonuniform distribution of the field intensity in the slot long-axis direction. This is a great cause of preventing a uniform distribution of an excited plasma in the long-axis direction of each slot.
This phenomenon is caused by the property that a plasma is easily excited in a central position along the longitudinal direction of each slot because the intensity of an electromagnetic wave as an excitation source is a maximum in this central position, and by the property that the excited plasma readily concentrates to have the smallest surface area in the form of a sphere. This plasma excited in the central position forms a region having a low spatial impedance in the center of the slot. This portion preferentially consumes energy. Also, the plasma functions as a shield to reduce the slot length, and a slot designed to have a length corresponding to a microwave emission portion actually functions as a slot having a length half the designed length. As a consequence, no microwave is emitted outside the slot. By these two factors, a plasma is readily formed only in the center of a slot, and it is very difficult to excite a uniform plasma over the slot, as shown in FIG. 13B.
The present invention has been made to solve the above problems, and has as its object to control the excited state distribution of a laser gas in the longitudinal direction of each slot in a slot array structure so as to uniformly excite the laser gas over the length of each slot with minimum energy loss.
According to the present invention, there is provided a laser oscillating apparatus for generating a laser beam by introducing an electromagnetic wave into a laser tube filled with a laser gas through a slot formed in a waveguide wall characterized by comprising an energy supplying portion for supplying energy to the laser gas to control an excited state distribution of the laser gas independently of introducing the electromagnetic wave into the laser tube through the slot.
In the laser oscillating apparatus according to the present invention, the energy supplying portion preferably supplies energy to the laser gas so as to substantially uniformly excite the laser gas along the slot.
In the laser oscillating apparatus according to the present invention, the energy supplying portion preferably includes, near the slot, an electrode for supplying energy to the laser gas.
In the laser oscillating apparatus according to the present invention, the energy supplying portion preferably ionizes the laser gas by supplying energy to the laser gas using the electrode.
In the laser oscillating apparatus according to the present invention, it is preferable that the energy supplying portion preliminarily ionize the laser gas before supplying an electromagnetic wave through the waveguide.
In the laser oscillating apparatus according to the present invention, the electrode preferably supplies energy to the laser gas to make an ionization density near an end portion of the slot higher than an ionization density near a central portion of the slot.
In the laser oscillating apparatus according to the present invention, the energy supplying portion preferably includes, near the slot, at least one pair of electrodes for supplying energy to the laser gas, and the at least one pair of electrodes are arranged to make an ionization density near an end portion of the slot higher than an ionization density near a central portion of the slot.
In the laser oscillating apparatus according to the present invention, it is preferable that the energy supplying portion include at least one pair of electrodes for supplying energy to the laser gas, and the at least one pair of electrodes be shaped to make an ionization density near an end portion of the slot higher than an ionization density near a central portion of the slot.
In the laser oscillating apparatus according to the present invention, it is preferable that the energy supplying portion include, near the slot, a plurality of electrode pairs for supplying energy to the laser gas, and a current with a current density higher than a current density of a current supplied between the electrode pair near a central portion of the slot be supplied between the electrode pair near an end portion of the slot.
In the laser oscillating apparatus according to the present invention, it is preferable that the energy supplying portion include, near the slot, a plurality of electrode pairs for supplying energy to the laser gas, and the plurality of electrode pairs concentrate near an end portion of the slot.
In the laser oscillating apparatus according to the present invention, it is preferable that the energy supplying portion include, along the slot, one pair of electrodes for supplying energy to the laser gas, and each of the pair of electrode be wider near an end portion of the slot than near a central portion of the slot.
In the laser oscillating apparatus according to the present invention, it is preferable that the energy supplying portion include, near the slot, a plurality of electrode pairs for supplying energy to the laser gas, and the electrode pair near an end portion of the slot be located nearer to the slot than the electrode pair near a central portion of the slot.
In the laser oscillating apparatus according to the present invention, the energy supplying portion preferably supplies energy to the laser gas to make an ionization density near an end portion of the slot higher than an ionization density near a central portion of the slot.
In the laser oscillating apparatus according to the present invention, the energy supplying portion preferably includes a light source for irradiating a portion near the slot with light.
In the laser oscillating apparatus according to the present invention, it is preferable that the waveguide be placed between the light source and the laser tube, and one surface of the waveguide have a window transmitting light from the light source.
In the laser oscillating apparatus according to the present invention, the surface of the waveguide in which the window is preferably formed opposes a surface of the waveguide in which the slot is formed.
In the laser oscillating apparatus according to the present invention, the window preferably has a size that disables the electromagnetic wave from passing through.
In the laser oscillating apparatus according to the present invention, the energy supplying portion preferably further includes a reflecting mirror for reflecting light from the light source toward a portion near the slot.
In the laser oscillating apparatus according to the present invention, the light source preferably generate ultraviolet light.
In the laser oscillating apparatus according to the present invention, the plurality of slots are preferably formed in the waveguide.
In the laser oscillating apparatus according to the present invention, the plurality of slots are preferably formed along a longitudinal direction of the waveguide.
In the laser oscillating apparatus according to the present invention, the energy supplying portion is preferably provided for each slot.
According to the present invention, there is provided an exposure apparatus including a laser oscillating unit, an illumination optical system for generating illuminating light for illuminating a mask by using a laser light supplied from the laser oscillating unit, and a projection optical system for projecting a pattern of the mask illuminated with the illuminating light generated by the optical system onto a substrate, characterized in that the laser oscillating unit generates the laser beam by introducing an electromagnetic wave into a laser tube filled with a laser gas through a slot formed in a waveguide wall, and the unit comprises an energy supplying portion for supplying energy to the laser gas to control an excited state distribution of the laser gas independently of introducing the electromagnetic wave into the laser tube through the slot.
According to the present invention, there is provided a device manufacturing method characterized by comprising the steps of coating a substrate with a photosensitive material, exposing a pattern on the substrate coated with the photosensitive material by using the exposure apparatus defined in claim 23, and developing the pattern on the exposed substrate.